Messages in this thread | | | Subject | Re: [PATCH v2] vfio/type1: avoid redundant PageReserved checking | From | Ben Luo <> | Date | Fri, 30 Aug 2019 00:58:22 +0800 |
| |
在 2019/8/28 下午11:55, Alex Williamson 写道: > On Wed, 28 Aug 2019 12:28:04 +0800 > Ben Luo <luoben@linux.alibaba.com> wrote: > >> currently, if the page is not a tail of compound page, it will be >> checked twice for the same thing. >> >> Signed-off-by: Ben Luo <luoben@linux.alibaba.com> >> --- >> drivers/vfio/vfio_iommu_type1.c | 3 +-- >> 1 file changed, 1 insertion(+), 2 deletions(-) >> >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c >> index 054391f..d0f7346 100644 >> --- a/drivers/vfio/vfio_iommu_type1.c >> +++ b/drivers/vfio/vfio_iommu_type1.c >> @@ -291,11 +291,10 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async) >> static bool is_invalid_reserved_pfn(unsigned long pfn) >> { >> if (pfn_valid(pfn)) { >> - bool reserved; >> struct page *tail = pfn_to_page(pfn); >> struct page *head = compound_head(tail); >> - reserved = !!(PageReserved(head)); >> if (head != tail) { >> + bool reserved = PageReserved(head); >> /* >> * "head" is not a dangling pointer >> * (compound_head takes care of that) > Thinking more about this, the code here was originally just a copy of > kvm_is_mmio_pfn() which was simplified in v3.12 with the commit below. > Should we instead do the same thing here? Thanks, > > Alex ok, and kvm_is_mmio_pfn() has also been updated since then, I will take a look at that and compose a new patch > > commit 11feeb498086a3a5907b8148bdf1786a9b18fc55 > Author: Andrea Arcangeli <aarcange@redhat.com> > Date: Thu Jul 25 03:04:38 2013 +0200 > > kvm: optimize away THP checks in kvm_is_mmio_pfn() > > The checks on PG_reserved in the page structure on head and tail pages > aren't necessary because split_huge_page wouldn't transfer the > PG_reserved bit from head to tail anyway. > > This was a forward-thinking check done in the case PageReserved was > set by a driver-owned page mapped in userland with something like > remap_pfn_range in a VM_PFNMAP region, but using hugepmds (not > possible right now). It was meant to be very safe, but it's overkill > as it's unlikely split_huge_page could ever run without the driver > noticing and tearing down the hugepage itself. > > And if a driver in the future will really want to map a reserved > hugepage in userland using an huge pmd it should simply take care of > marking all subpages reserved too to keep KVM safe. This of course > would require such a hypothetical driver to tear down the huge pmd > itself and splitting the hugepage itself, instead of relaying on > split_huge_page, but that sounds very reasonable, especially > considering split_huge_page wouldn't currently transfer the reserved > bit anyway. > > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> > Signed-off-by: Gleb Natapov <gleb@redhat.com> > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index d2836788561e..0fc25aed79a8 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -102,28 +102,8 @@ static bool largepages_enabled = true; > > bool kvm_is_mmio_pfn(pfn_t pfn) > { > - if (pfn_valid(pfn)) { > - int reserved; > - struct page *tail = pfn_to_page(pfn); > - struct page *head = compound_trans_head(tail); > - reserved = PageReserved(head); > - if (head != tail) { > - /* > - * "head" is not a dangling pointer > - * (compound_trans_head takes care of that) > - * but the hugepage may have been splitted > - * from under us (and we may not hold a > - * reference count on the head page so it can > - * be reused before we run PageReferenced), so > - * we've to check PageTail before returning > - * what we just read. > - */ > - smp_rmb(); > - if (PageTail(tail)) > - return reserved; > - } > - return PageReserved(tail); > - } > + if (pfn_valid(pfn)) > + return PageReserved(pfn_to_page(pfn)); > > return true; > }
| |